Gas sensor

ABSTRACT

An electro-chemical sensor for the measurement of concentrations of gas or vapor in accordance with the limiting current principle and comprising an electrolytic cell having a sensing electrode, a counter electrode and an intervening body of electrolyte and also a restriction to the rate of access of gas or vapor to the sensing electrode. The improvement comprises said restriction being in the form of a gas-phase diffusion barrier including at least one narrow passage for the diffusion of gas. The gas-phase diffusion barrier may be in the form of a porous body, of which the permeability of the body may be more than twenty times greater than the diffusibility, but is preferably less than ten times the diffusibility and more preferably not more than 10% greater than the diffusibility. Alternatively the gas-phase diffusion barrier may be defined by a capillary. The capillary may be of plastics material and may be enclosed within an outer metal sheath of lesser co-efficient of expansion. The capillary may be connected in series with a porous body.

This invention relates to electro-chemical gas sensors in which the gasor vapour to be sensed is caused to react at one electrode of anelectro-chemical cell, which also includes a counter electrode and anintervening body of electrolyte, in such a way that the current throughthe cell is a function of the partial pressure of the gas or vapour tobe sensed. The principle will be described in relation to the sensing ofoxygen although it is to be understood that it is applicable to any gasor vapour which can be electro-chemically reacted in this way. Gases orvapours such as, for example, oxygen and nitrogen oxides which areelectro-chemically reducible, are sensed at the cathode, while thosewhich are electro-chemically oxidisible, such as carbon monoxide,sulphur dioxide and hydrogen sulphide are sensed at the anode.

In practice oxygen is electro-chemically reduced at an oxygen cathode,to which the rate of access of oxygen is restricted, for example by adiffusion barrier, under conditions such that the electrode is operatingin the so-called limiting current region, illustrated by the shaded areaof FIG. 1 of the accompanying drawings, which is a graph of currentdensity plotted against polarizing potential and represents a typicaloxygen-electrode polarization curve in which the limiting current isindicated as i_(L). Under limiting current conditions the concentrationof oxygen at the electrode surface is essentially zero. The limitingcurrent, i_(L) will then be proportional to the flux of oxygen, whichwill be a function of the partial pressure of oxygen in the gas beingsensed. The relation between the partial pressure of oxygen and the fluxof oxygen and hence the limiting current will be determined by the lawsgoverning the permeation of oxygen across the diffusion barrier. Tocomplete the sensing cell it is provided with an electrolyte and acounter electrode i.e., an anode, at which an oxidation reaction willoccur.

The choice of anode material and mode of operation of an oxygen sensorof this type may be considered against the background of the curve shownin FIG. 1. The oxygen electrode must be polarised sufficiently to bewell within the limiting current region, where the value of the limitingcurrent is insensitive to potential, but it must not be polarised belowthe hydrogen potential where it can evolve hydrogen giving a falsereading and problems with hydrogen gas bubbles. If the anode material ischosen so that its operating potential is within the shaded region ofFIG. 1, then sufficient volts are available for the sensor to beself-powered and it can be operated simply with a suitable load resistorbetween anode and cathode, the current being measured with a volt meter.

If the anode has an operating potential above the shaded region, then anexternal power source will be required to polarise the oxygen cathodesufficiently into the limiting current region. The applied voltage mustbe kept below the value that would polarise the cathode into thehydrogen evolution region. The use of an anode material with anoperating potential below the shaded region is to be avoided, sincefirstly a constant voltage load circuit is required to keep the cathodepotential in the required safe region and secondly an anode is liable toself-discharge with the production of hydrogen if the sensor is on opencircuit. If the gas to be sensed is a reducing gas parallelconsiderations apply, but here the sensing electrode is now the anode,which should be polarised well into the limiting current region, but notso far that the oxygen evolution potential is reached.

Known types of gas sensors working on these general principles achievethe necessary restriction to the access of gas to the sensing electrodeby the provision of a solid membrane between the body of the gas to betested and the sensing electrode so that the flux of gas is restrictedby the diffusion rate of gas in solution in the solid membrane. Examplesof such solid membranes that have been used are films ofpolytetrafluoroethylene, polyethylene and silicone rubber. An example ofa sensor using such a membrane is that described in S.M.R.E. Digest. GasDetection 1. 1972.

The use of solid membrane films has the disadvantages that very thinmembranes have to be used to achieve practical current levels and moreimportantly that the resulting sensor has an extremely high temperatureco-efficient, which may be as high as 2% to 3% per degree centigrade.This means that provision must be made for temperature compensation andthis is difficult to achieve accurately and reliably. This hightemperature coefficient is a fundamental consequence of the process oftransporting gas in solution through a solid material, being associatedwith the high activation energy required for this purpose.

According to the present invention a sensor of the type just described,operating on the limiting current principle for the detection of oxygenor other gas or vapour includes a gas phase diffusion barrier, that isto say a barrier through which the significant transport process toprovide the necessary controlled restriction to the access of gas to thesensing electrode is one of gaseous diffusion rather than of diffusionof the gas in solution. For this purpose, one or more narrow gaspassages need to be provided through an otherwise solid barrier and thebarrier may thus be either in the form of a porous sheet, plug ormembrane or in the form of a simple capillary tube or a combination ofthese two alternatives arranged in series with one another.

We have found that with a barrier of this type diffusion and theresulting sensor signal varies so little with temperature that, for manypractical purposes, no temperature compensation is necessary, so thatthe arrangement as a whole is thus simplified and greater accuracy andreliability results.

We have also found that with a barrier of this type the sensor measurescomposition, e.g. percentage oxygen in the gas, and that the resultingsensor signal at a given composition is independent of pressure. This isclearly an important advantage in any application where variations inpressure can occur and a measure of gas composition is required.

If the gas phase diffusion barrier has pores of sufficiently large sizesome gas transport may occur by bulk flow as well as by diffusion. Theeffect of this is to result in a non-linear variation of signal withcomposition. The non-linearity becomes increasingly noticeable as thepercentage of the gas being measured (e.g. oxygen) rises above about20%. Explanation for this effect can be found in Principles of UnitOperations. John Wiley and Sons Inc., 1966, page 118.

For many purposes this is not of practical importance, but may lead todifficulties when high concentrations need to be measured. Moreover, thebulk flow effect can also lead to the sensor signal being sensitive todraughts and modulations to the ambient pressure.

Preferably, therefore, these bulk flow effects are substantiallyeliminated by the use of porous membranes of limited pore size. Toprevent any appreciable bulk flow the equivalent pore size needs to benot greater than 1 micrometer and preferably appreciably lower thanthis, i.e., as low as or lower than 0.3 micrometers to 0.03 micrometers.Any particular porous body will have a range of pore sizes and poreshapes and a variation of pore shapes through the thickness of the body.As a consequence, the limitation of pore size is best determined interms of the limitation of bulk flow effects. A guide to this can beobtained as a relationship between the permeability (bulk flow plusdiffusion) and diffusibility (diffusion only). A restriction which isadequate to avoid any major effects of draughts is obtained if theformer is not more than twenty times as great as the latter, while toeliminate the effect of draughts altogether, the permeability needs tobe less than ten times as great as the diffusibility. To obtaineffective linearity the requirement is considerably more stringent andfor this purpose the permeability should not exceed the diffusibility bymore than 10%.

Examples of commercially available porous membrane materials whose usewill effectively eliminate the bulk flow effect are some grades ofporous "unsintered" polytetrafluorethylene tape and porous polycarbonatefilms available commercially under the Trade names "Nuclepore","Unipore", "Millipore".

It can be predicted theoretically and confirmed experimentally that whenusing a gas-phase diffusion barrier as so far described, there willstill be a very small residual temperature co-efficient. If the barrieris defined by a capillary, even this small co-efficient can be virtuallyeliminated if the capillary is of plastics material and is enclosedwithin an outer metal sheath of lesser co-efficient of expansion. If,for example, the temperature rises the plastics material expands morethan the constraining metal tube so that the capillary passage throughthe plastics tube tends to constrict and lengthen. Thus, for example,with the use of a 5 mm long vinyl capillary (temperature co-efficient ofexpansion about 70 × 10⁻⁶ per degree centigrade) of 3 mm outsidediameter and 0.25 mm bore, mounted in a stainless steel outer tube(temperature co-efficient of expansion about 10 × 10⁻⁶ per degreecentigrade), the differential expansion effect will almost exactlycompensate for a 0.17 per cent per degree centigrade temperatureco-efficient. Although this represents a valuable refinement, accuracyof this order is not often required and this form of construction willnot be described in further detail.

In sensors of the type described the operating life is governedprimarily by the amount of counter electrode material (anode in the caseof the oxygen sensor) in the cell and the total cell current. In theinterests of long life and small size it is therefore worthwhile toreduce the total cell current as far as practical, a value of less than10 mA usually being desirable. As a guide the total current for anoxygen sensor with a porous membrane operating in ambient air will begiven approximately by i_(L) = 0.76 Aθ/τL where i_(L) is the totalsensor current in amps.

A is the area in cm²

θ is the fractional porosity

τ is the tortuosity factor

L is the thickness in cm

Undue increases in the values of τ and L adversely affect the responsetime, so that attention needs to be directed to the values of A and θ.In the case of a Nuclepore membrane, the manufacturing process is suchthat the porosity of the membrane can be controlled over a fairly widerange, but in other cases, such as with commercially available porousunsintered PTFE, the porosity is inconveniently high. In such cases, itis possible to reduce the total porosity by selective filling of thepores, for example by successive impregnations with solutions of wax orresins, but this leads to difficulty in obtaining even properties overthe whole effective area of membrane.

With a porous unsintered PTFE membrane a preferred way of reducingporosity is by pre-pressing the membrane before assembly into thesensor. We have found that a pressure of about 2.5 tons per square inchis suitable.

Details of this and of other factors involved and of electro-chemicalsensors in accordance with the invention will now be described, by wayof example, with reference to FIGS. 2 to 7 of the accompanying drawings,in which:

FIG. 2 is a graph in which the sensor current and response time areplotted against pressing pressure;

FIG. 3 is a graph in which capillary length and sensor current areplotted against capillary diameter;

FIG. 4 is an exploded perspective view of one construction of detectorcell in accordance with the invention;

FIG. 5 is a detailed cross sectional view showing part of a modificationof the construction of FIG. 4;

FIG. 6 is a view similar to FIG. 4 showing another form of constructionof cells; and

FIG. 7 is a graph showing total sensor current plotted against percentage oxygen.

FIG. 2 shows the effect of pressing pressure on sensor current andresponse time when using an 0.008 inch thick PTFE tape which has aninitial porosity of about 26%, as supplied by W. L. Gore and Associates(UK) Limited, for cable insulation. As can be seen, higher pressingpressures will tend to unduly extend the response time of the resultingsensor. To ensure a suitably low current from the sensor it is stillnecessary to restrict the diffusion area and, for example, acommercially available porous PTFE tape pressed as described aboveshould be masked to limit the diffusion area to the equivalent of 1 to1.5 millimeters diameter to reduce the sensor current to the order of 1to 2 mA/cm ambient air.

The use of a pressed porous PTFE membrane in the manner describedeffectively eliminates bulk flow and sensors made in this way have givena linear response up to 100% oxygen.

If the gas phase diffusion barrier takes the form of a capillary thenthis can be made to provide the controlling restriction to diffusion. Asa guide to the choice of capillary size the current on ambient air willbe approximately 0.6 d² /L amps where d and L are the diameter andlength of the capillary respectively in centimeters. Moreover, it isfound that the response time of the sensor increases with the length ofcapillary used, so that in choosing a suitable combination of d and Lfor any specific purpose, this factor needs to be taken into account.

FIG. 3 shows the effect of variations in length and diameter of thecapillary as a function of sensor current. Whatever the proportions ofthe capillary, it is preferable to include a low pore size membrane inseries with the capillary in order to reduce the bulk flow effectdiscussed earlier. If a very short capillary is used, it might normallybe regarded as constituting a narrow orifice or jet. For presentpurposes, however, the term "capillary" is used, regardless of thelength.

Although the parameters are quoted in terms of a capillary of circularcross section, capillaries of other cross sectional shapes may be used,and it is not essential for the cross sectional shape and area to beconstant throughout the length of capillary. Neither is it essential forthe capillary to be straight: it can, for example, contain bends or becoiled or folded.

These size considerations have been discussed in the context ofoperation around normal oxygen concentrations in air. For themeasurement of low concentrations of gas the masking hole and capillarysizes may be altered to result in a signal of appropriate level for theapplication.

The sensing electrode, the cathode in the case of an oxygen sensor,should have sufficient activity that it does not itself constitute alimitation to the current, this limitation being controlled by theporous membrane and/or the capillary where this is incorporated. Thereare many examples of active oxygen electrodes that have been developedfor fuel cells and metal-air batteries. A suitable cathode may bereadily made by mixing a finely divided catalyst such as platinum,silver, silverised graphite or high surface area graphite with PTFEsuspension and applying this to a suitable current collector such as anickel gauze, followed by a drying and curing treatment. In order toensure that there is no seepage of electrolyte through the electrode itis desirable to apply a further layer of porous hydrophobic material,such as PTFE, to the gas side of the electrode. Where the porousdiffusion barrier membrane is itself hydrophobic, for example when it isporous PTFE, this will serve as the waterproofing layer if it is presseddirectly on to the electrode.

In other cases it is desirable to mount the porous membrane separatelyfrom the electrode with a gap between the two, for example if themembrane is Nuclepore, which is hydrophilic, or for reasons of easierfabrication.

Similarly when a capillary is used it is preferable to have a shallowcavity, or relatively larger area than the cross sectional area of thecapillary between the inner end of the capillary and the electrode. Thisenables the current to be spread over sufficient electrode area toresult in a low current density that will not produce limiting effectsat the electrode itself.

Suitable electolytes for use in the sensor include potassium hydroxide,potassium carbonate, sodium hydroxide, potassium phosphate, potassiumcitrate and potassium borate. The electrolyte may be mixed with agelling agent such as carboxy methyl cellulose.

Most of the factors governing the choice of anode materials for anoxygen sensor in accordance with the invention are the same as werediscussed earlier. Preferred materials with potentials in the shadedregion of FIG. 1 include cadmium, lead, bismuth and copper. The anodematerial may be in any suitable form such as foil, corrugated orperforated foil, mesh, pressed powder or it may be plated on to a foilor mesh current collector. When the metal to be used is lead, this mayconveniently be in the form of lead wool. A particular advantage can beobtained with cadmium plated from a fluoroborate bath containingcaffeine, resorcinol and pepsin as additives. Such anode when used in anoxygen sensor in accordance with the invention and with potassiumcarbonate electrolyte may achieve utilisations of the cadmium of closeto 100%.

If the anode material, such as lead or cadmium, is plated on to a metal,which is more noble, such as copper, then an additional advantage can beobtained, namely to make possible a check on the state of discharge ofthe sensor. When the plated material is exhausted the open circuitvoltage of the sensor will drop appreciably, so that this stage may bechecked by measurement. The sensor will, however, continue to functionsince the copper will now function as the anode material.

Sensors in accordance with the invention may be constructed in a varietyof shapes and sizes to best fit the application concerned. For examplethe sensors may be made in a cylindrical shape like a conventionalprimary battery with the membrane and sensing electrode at one end, orthey may be made in an annular or "wrap-around" design, in which themembrane and sensing electrode are attached to an inner tube, aroundwhich is an annular container holding the electrolyte and counterelectrode. An annular design is particularly suitable for incorporationin a flow system.

FIG. 4 is an exploded perspective view of one form of oxygen sensor inaccordance with the invention. The cell is contained within anickel-plated can 11, the bottom portion of which holds an anode 14constituted by granulated lead and potassium carbonate electrolyte ofsufficient volume to have a free surface at 15 just above the surface ofthe anode.

A fibrous disc 16 which is fitted immediately above the surface of theelectrolyte acts as a wicking separator and contacts the cathode of thecell 18, which operates as the oxygen electrode. The cathode consists ofnickel gauze covered with a paste of silverised graphite catalyst. Asmall bolt 20 extends upwardly from the cathode 18 and through thesuperimposed components to a terminal tag 21 constituting the cathodeconnection of the cell, this tag being held in position by a nut 22.

The cathode is waterproofed by a layer 24 of unsintered porous PTFE tapepressed into contact with the electrode to form a unitary assembly. Thisassembly is secured by an adhesive seal 26 to an end cap 28 formed witha capillary 30. The construction is such as to leave a narrow gapbetween the inner end of the capillary 30 and the top of the electrodeassembly, as indicated by the circle 31. The end cap is provided with anO-ring 32 which fits into the flared mouth 33 of the can 11 so as toform a tight sealing fit. Finally, the outer end of the capillary 30 iscovered by a porous PTFE membrane 34 glued to the upper surface of theend cap 28 in order to reduce transport of oxygen by bulk flow. Thenickel-plated can 11 constitutes the anode connection and, in use, thecell is loaded with a resistor to bring the sensor current into thelimiting current region.

FIG. 5 which is a detailed view to an enlarged scale shows, in crosssection, the central portion of an alternative form of end cap. As canbe seen, this end cap, also identified as 28, has a relatively largecentral hole 36, e.g. of 1 mm diameter, rather than a capillary, thishole merely serving as a mask to limit the effective area of thediffusion barrier which is constituted by a porous PTFE membrane 38which takes the place of the waterproofing layer 24 shown in FIG. 4. Asbefore, the end cap 28 is glued to the PTFE membrane by a layer ofadhesive sealant 26.

EXAMPLE I

An oxygen sensor in accordance with FIG. 5, the remainder of theconstruction being in accordance with FIG. 4, was made as follows.

The cathode 18 was made by spreading a well mixed paste of silverisedgraphite catalyst and PTFE suspension (10:3 ratio of catalyst to PTFE byweight) on to 80 mesh nickel gauze. This was allowed to dry and was thencured at 200° C for one hour. The gas phase diffusion barrier 38comprising commercial porous unsintered PTFE tape (0.008 inch thickunstretched insulating tape obtained from W. L. Gore and Associates (UK)Limited) was then pressed at 2.5 tons per square inch on to this cathodeto form an integral assembly of the diffusion barrier and cathode. Thepressing operation also reduced the pore-size, as previously described.Measurement showed that the ratio of permeability to diffusibility was1.08:1 so as to give an effectively linear response.

The end cap 28 was made of polymethacrylate and the hole 36 had adiameter of 1 mm to restrict the diffusion area. The remaining detailswere as already described with reference to FIG. 4 and the electrolytewas a four molar solution of potassium carbonate. After loading with a 1ohm resistor to bring the sensor into the limiting current region, thesensor gave a signal in ambient air of 1.4mA and responded to differentoxygen concentrations in the manner shown in FIG. 7. No change in thesignal in ambient air was detected over the temperature range of 0 to50° C. The response time of the sensor for an 80% change when placed inpure nitrogen was eight seconds.

EXAMPLE II

An oxygen sensor in accordance with FIG. 4 was made up as follows.

The capillary 30 was formed through the end cap 28 by drilling a holehaving a diameter of 0.34 mm and a length of 7 mm. The cathode 18 wasmade as in Example I and was waterproofed by means of the layer 24 bypressing unsintered porous PTFE tape of the same grade as described inExample I on to one side of the electrode, using a pressing pressure of1 ton per square inch. This was sealed to the end cap 28 in such a waythat the narrow gap denoted by the circle 31 in FIG. 4 had a depth ofabout 0.5 mm and a diameter of 1 cm.

The membrane 34 fitted over the outer end of the capillary 30 was formedfrom the same grade of porous PTFE tape referred to previously, the poresize without any pre-pressing being sufficiently small to reduce thetransport of oxygen by bulk flow to an extent at which the permeabilitywas four times greater than the diffusibility. The resulting sensor wasinsensitive to draughts although its response was not completely linearat oxygen concentrations above 25%. When loaded as in Example I thissensor gave a signal in ambient air of 0.90 mA. The change in signalbetween ambient temperature and -10° C was less than 3%. There was nodetectable change in signal after the air pressure had been raised tothree atmospheres.

FIG. 6 shows an alternative construction of sensor, the modificationsbeing primarily to the end cap and the cathode connection. Insofar asparts correspond to those included in FIG. 4 or FIG. 5, they areindicated by the same reference numerals. Thus a nickel-plated can 11holds a wool lead anode 14 and electrolyte 15, on top of which is awicking separator 16. In this construction, the can 11 is formed with arill 40 at the level of the wicking separator 16 and the portion of thecan above the rill 40 is insulated by a nylon sealing grommet 41. Theassembly of cathode 18 and PTFE membrane 24 is constructed in the sameway as in FIGS. 4 and 5, the membrane merely serving a water-proofingfunction as in FIG. 4 rather than acting as a diffusion barrier as inFIG. 5.

In this construction the cathode connection is defined by a thin silverstrip 43 which is in contact with the lower face of the cathode 18 andthe end portions of which extend upwardly between the grommet 41 and theend cap 28. In this modified construction, the capillary is defined by alength 44 of commercial stainless steel hypodermic tubing mounted in athick-walled sleeve 45 of vinyl plastics which is itself containedwithin an outer stainless steel tube 46 fitted into the end cap 48. Thecapillary forms the gas-phase diffusion barrier and bulk flow effectsare restricted by a membrane 34 as in FIG. 4, which is held in positionby a top cap 48 fitting over the outside of the tube 46. The end 48 isheld in position by crimping of the top rim 50 of the can 11.

EXAMPLE III

An oxygen sensor in accordance with FIG. 6 was made up as follows.

The lower part of the sensor was substantially in accordance withExample I except that five molar sodium hydroxide was used aselectrolyte. In particular, the assembly of cathode 18 and waterproofingmembrane 24 was constructed as previously. The hypodermic tubingdefining the capillary 44 had a length of 5 mm, a bore of 0.3 mm and anoutside diameter of 0.56 mm. The end cap 48 was made of nickel-platedmild steel which was thus in electrical connection with the cathodethrough the contact strip 43, as a result of pressure contact with thestrip produced by the crimping of the top rim. The cathode connectionwas spot-welded to the top cap and the anode connection was then takendirectly from the can 11.

The membrane 34 had the same characteristics as in Example II, with anopen area of 0.07 square cms defined by a 3 mm diameter hole in top cap48. When the cell was loaded as in Example I this construction of sensorgave a signal in ambient air of 1.08 mA, which was insensitive todraught.

In the example just described with reference to FIG. 6, the capillary isformed in a separate unit fitted into the end cap and this isparticularly advantageous. Various modifications to the detailedconstruction shown in FIG. 6 are possible. Thus a thickwalled capillaryof either metal or plastics may itself form the unit fitted into the endcap and this may be enclosed within an outer metal sheath to provide theadditional temperature compensation referred to previously.

EXAMPLE IV

An oxygen sensor was made up exactly as in Example III, except that themembrane 34 consisted of a polycarbonate film available under theTrademark "Unipore" from BioRad Laboratories of Richmond, California,U.S.A. This film had a nominal pore size of 0.03 micrometers and apermeability less than 0.002 per cent greater than its diffusibility.This sensor gave a linear response against percentage oxygen up to 100per cent oxygen.

Any of the sensors so far described may be combined with electroniccircuits to give an audible or visible warning when the percentage ofgas to be sensed reaches or falls to a particular level and, inaddition, an electronic feedback circuit may be included for holding thevoltage of the cell at a constant value such that the current is withinthe limiting current region.

We claim:
 1. In an electro-chemical sensor for the measurement of concentrations of gas or vapour in accordance with the limiting current principle, said sensor comprising an electrolytic cell having a sensing electrode, a counter electrode and an intervening body of electrolyte and also including means restricting the rate of access of gas or vapour to said sensing electrode, the improvement wherein said restricting means comprises a gas-phase diffusion barrier defining at least one narrow passage for the diffusion of gas, said gasphase diffusion barrier including structure formed with capillary passage means, said structure being made of plastics material and further including an outer metal sheath being of lesser co-efficient of expansion than said plastics material of said structure.
 2. In an electro-chemical sensor for the measurement of concentrations of gas or vapour in accordance with the limiting current principle, said sensor comprising an electrolytic cell having a sensing electrode, a counter electrode and an intervening body of electrolyte and also including means restricting the rate of access of gas or vapour to said sensing electrode, the improvement wherein said restricting means comprises a porous body in series with capillary passage means, the porous body and the capillary passage means cooperating to provide a gas-phase diffusion barrier defining at least one narrow passage for the diffusion of the gas being sensed so that the gas being sensed remains in the gas phase throughout the process of transport from the gas mixture under test to the sensing electrode.
 3. An electro-chemical sensor according to claim 2, said sensor being adapted for use with an electro-chemically reducible gas and in which said counter electrode is a consumable metal anode which is more noble than hydrogen.
 4. An electro-chemical sensor according to claim 3 in which said anode is made of lead.
 5. An electro-chemical sensor according to claim 3 in which said anode is made of cadmium.
 6. An electro-chemical oxygen sensor according to claim 3 in which said sensing electrode is a cathode including a nickel mesh.
 7. An electro-chemical sensor according to claim 2 wherein said porous body has a permeability not more than twenty times greater than its diffusability.
 8. An electro-chemical sensor according to claim 2 wherein said porous body has a permeability which is less than ten times the diffusability thereof.
 9. An electro-chemical sensor according to claim 2 wherein said porous body has a permeability which is not more than 10% greater than the diffusability thereof.
 10. An electro-chemical sensor according to claim 2 additionally including a porous membrane barrier in series with the restricting means and interposed between the gas mixture under test and the sensing electrode.
 11. An electro-chemical sensor according to claim 2 wherein the porous body is arranged upstream from the capillary passage means along a path of flow normally followed by the gas or vapour in reaching said sensing electrode.
 12. An electro-chemical sensor according to claim 2 wherein the sensing electrode together with the restricting means are fixed and sealed into a supporting housing. 